In radiotelephone communication systems, proper operation generally requires the transmission of control signals along with the desired voice or data signal. The control signals are used, for example, to page or call a mobile subscriber unit, and to perform a handshaking procedure between the mobile subscriber unit and the remote base station. Two signal types that play an important role in the handshaking procedure of the cellular Advanced Mobile Phone System (AMPS) are the Supervisory Audio Tone (SAT) signal and the Supervisory Tone (ST) signal.
The SAT signal generally serves two functions. First, it enables cellular base stations in one particular cluster of cells to be distinguished from base stations in a neighboring cluster of cells that use the same channel frequencies. A mobile subscriber unit is therefore constrained to operate with the base stations in its vicinity even though it could also be receiving an adjacent cluster signal because the adjacent cluster signal will have an incorrect SAT. Second, the SAT signal makes a closed loop identification between the mobile subscriber unit and the base station. If the SAT is lost during a call, the base station a timer. If the SAT signal is not received before the time expires, the call is terminated.
Some of the functions of the ST signal are to confirm a handover request, to send a request for additional services, and to send confirmation of an alert after the mobile subscriber unit has been alerted.
The AMPS cellular telephone standard requires a 10 Kbit/sec data signal to be intermittently transmitted by the subscriber unit. The data signal is Manchester encoded to produce a High Speed Data (HSD) signal at a sample rate of 160 Ksamples/sec. The ST signal is derived from the HSD signal and therefore also has a sample rate of 160 Ksamples/sec. When the data is not being transmitted, the audio (voice) signal can be transmitted. The sample rate of the audio signal after digital processing is 128 Ksamples/sec. The supervisory audio tone (SAT) has a sample rate of 128 Ksamples/sec.
In addition, during certain transmission times, it is necessary to simultaneously send the supervisory audio tone (SAT) and the supervisory tone (ST). Therefore, the transmission of signals in the AMPS cellular standard generally involves three conditions. Condition one is when the audio signal (128 Ksamples/sec) is simultaneously transmitted with the SAT signal (128 Ksamples/sec). Condition two is when the HSD signal (160 Ksamples/sec) is transmitted. Condition three is when the ST signal (160 Ksamples/sec) is simultaneously transmitted with the SAT signal (128 Ksamples/sec).
In order to simultaneously transmit the ST and SAT signals, the ST signal is summed with the SAT signal. The ST signal and the SAT signal are routed to a common modulator circuit, so it is important that the sampling rate of the two signals be substantially equal at the modulator interface.
One possible method to get the sampling rates of the ST and the SAT signals to be substantially equal is to convert the SAT signal from 128 Ksamples/sec to 160 Ksamples/sec. This approach, however, would still result in signals interfacing with the modulator at more than one sample rate. For example, during condition one transmission, the audio signal would be at 128 Ksamples/sec and the SAT signal would be at 160 Ksamples/sec. However, this approach would lead to more circuit complexity due to the need to interface the modulator to two sampling rates.
Thus, converting the SAT signal sample rate would require extra circuitry to account for the signals at the two sample rates and would not result in the most reduced complexity circuitry that is optimized to reduce the die area of an integrated circuit (IC). Further, the approach would not yield the lowest current drain solution. A reduction in IC area would result in an overall reduction in the size and cost of a radiotelephone that incorporates the IC, and a reducing the size of a radiotelephone is very desirable feature. Lowering the current drain of radiotelephone circuitry is also desirable because lower current drain translates into a longer period of sustainable operation. Therefore, there is a need to convert the HSD signal, including the ST signal, from a sample rate of 160 Ksamples/sec to 128 Ksamples/sec. This would result in a single sample rate being presented to the modulator interface during all three conditions of transmission.
One possible method to achieve the sample rate conversion is through the use of a linear decimator based on a Finite-duration Impulse Response (FIR) filter. This conventional approach, however, results in a complex design which does not result in the most reduced circuit complexity or the lowest current drain. Prior art methods require an additional high-speed clock, which further increases current drain, circuit complexity, and required silicon area of an integrated circuit. Much of the prior art circuitry also operates at the high clock speed, thus further increasing current drain. Therefore, there is a need to accomplish the sample rate conversion with a minimal amount of hardware in order to not have a significant impact on the size and cost of a radiotelephone mobile subscriber unit. There is a further need to perform the sample rate conversion without the need for an additional high speed clock and its associated circuitry.